What subscripts are needed to complete the formula unit li_n_? 1,1 3,1 1,3 3,3

In this problem we need to obtain a molecule with a neutral charge that also satisfies the noble gas configuration. Lithium (Li) is a group 1 metal so it needs to lose an electron to achieve noble gas configuration. The loss of an electron will give it a (+1) charge. In contrast, Nitrogen (N) is a group 5 element and needs to gain 3 electrons to achieve noble gas, hence it has a charge of (-3) charge. A neutral molecule has the sum of  oxidation states of all atoms equal to zero. Hence 3 molecules of lithium and 1 molecule of nitrogen give a neutral molecule.

Therefore 3,1 is the correct answer,

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the atomic mass is the bottom number on the element.

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